The present invention relates to finely divided precipitated silicate pigments and, more particularly, to novel modified sodium alumino silicate pigments which are chemically analagous to zeolites but which have certain unique properties and characteristics.
As is well known in the art, natural alkali metal zeolites such as natrolite and analcine are known to possess water softening properties but have limited usefulness as pigments. In recent years a number of artificial or synthetic zeolites containing sodium, aluminum and silicon oxides, in various proportions, have been proposed and synthesized. Examples of synthetic zeolites are disclosed in U.S. Pat. Nos. 2,882,243; 2,962,335; 2,996,358; 3,010,789; 3,011,869; and 3,012,853. To a large extent, known synthetic zeolites lack the essential characteristics of pigments and have found limited use in this field.
In this regard, however, there has been developed novel precipitated sodium alumino silicates that have particular use as pigments in paper coatings, as fillers in rubber compounds, in inks, paints, plastics, and the like. Such pigments are commercially available and are manufactured and sold under the trademark "Zeolex" by J. M. Huber Corporation. While "Zeolex" pigments contain the essential oxide components of natural and synthetic products, these pigments are precipitated products of alumina and .[.d.]. silica and contain chemically bound sodium and as such are readily distinguishable from gels and other synthetically produced materials. In general, these pigments are precipitated amorphous materials and are composed of particles having ultimate sizes of only a few hundredths of a micron in diameter which tend to cling together in clusters of up .Iadd.to .Iaddend.about 1 micron in diameter. In chemical composition, they typically contain Na.sub.2 O and Al.sub.2 O.sub.3 in a molar ratio near to 1, as in the range of about 0.8 to 1.2 mols of Na.sub.2 O per mol of Al.sub.2 O.sub.3 , together with silica which can be present in any of various selected concentrations ranging from as little as about 2.5 mol to as much as about 16 mols of SiO.sub.2 per mol of Al.sub.2 O.sub.3.
As will be discussed in more detail hereinafter, these pigments are advantageously prepared (see e.g., U.S. Pat. No. 2,739,073) by comingling dilute solutions of an alkali silicate and an aluminum salt, such as aluminum sulfate. In a preferred embodiment, an amount of water not greater than about one half of the volume of the solutions to be added is placed in a suitable reaction vessel provided with a strong agitator. The reactants, i.e., the dilute solutions of the alkali silicate and an aluminum salt, are then introduced in streams entering the water at widely spaced points. In U.S. Pat. No. 2,848,346 such pigments are prepared by comingling a dilute solution of an alkali and a dilute solution of a dispersion containing finely divided silica and aluminum sulfate. The latter is prepared by treating kaolin clay with a strong mineral acid, such as sulfuric acid.